**Question:** Which of the following describes the Westermark sign in Pulmonary Thromboembolism?

A. Increased oxygen saturation in the right atrium
B. Reduced oxygen saturation in the left atrium
C. Increased oxygen saturation in the pulmonary artery
D. Reduced oxygen saturation in the systemic artery

**Correct Answer:** A. Increased oxygen saturation in the right atrium

**Core Concept:**
The Westermark sign is a clinical finding associated with pulmonary thromboembolism (PTE), a life-threatening condition caused by blood clots blocking pulmonary arteries in the lungs. In this scenario, the clots obstruct blood flow and reduce oxygenation in the lungs.

**Why the Correct Answer is Right:**
In a patient with PTE, the clot in the pulmonary artery leads to hypoxia (low oxygen) due to reduced oxygenation in the lungs. The oxygen saturation in the right atrium increases as the oxygen-poor blood from the lungs flows into the right atrium. This phenomenon is known as the Westermark sign.

**Why Each Wrong Option is Incorrect:**

A. Reduced oxygen saturation in the right atrium (incorrect) - This would be expected in patients with right-sided heart failure, not PTE.
B. Reduced oxygen saturation in the left atrium (incorrect) - Oxygen saturation in the left atrium should be normal or slightly decreased due to preload reduction.
C. Increased oxygen saturation in the pulmonary artery (incorrect) - This sign would be observed in pulmonary hypertension, not PTE.
D. Reduced oxygen saturation in the systemic artery (incorrect) - Systemic arteries should have normal or decreased oxygen saturation in PTE, not increased.

**Clinical Pearl:**
The Westermark sign serves as a helpful diagnostic clue for PTE when combined with other clinical findings like tachycardia, decreased breath sounds, and elevated right atrial and pulmonary artery pressures. Early recognition and treatment of PTE can significantly improve patient outcomes.